Many mobile radiotelephones are designed to comply with the Narrowband Advanced Mobile Phone Service (NAMPS) standard. One part of this standard specifies that a Digital Supervisory Audio Tone will be transmitted along with the voice communications so that the processor in the mobile telephone and the processor in the Mobile Telephone Switching Office (MTSO) will know that the mobile telephone is still transmitting and receiving a good signal, has not gone out of range, and is not in a blind spot. The MTSO sends a DSAT signal to the mobile telephone, and the mobile telephone sends, back to the MTSO, the same DSAT signal that it received from the MTSO. The DSAT code is a 24 bit code, transmitted continuously at the rate of 200 NRZ bits per second (bps). Control information (instructions, requests, acknowledgements) is also sent at 200 NRZ bps or by Manchester encoding at 100 bps, and preempts the transmission of any DSAT signal. As long as a valid DSAT code is being received the processor knows that the communications are still good. If the received code is invalid--or does not exist for a predetermined period of time--then it is highly likely that communications have been disrupted and therefore the audio output of the mobile telephone should be muted, and the processors in the mobile telephone and the switching office should attempt to re-establish communications, possibly on a different channel or in a different service cell.
There is no convenient way of determining whether the first received bit is the first bit in the pattern, the last bit in the pattern, or somewhere in between. However, the pattern must be detected as soon as possible. If a valid DSAT pattern is not detected for a predetermined length of time then the processors will determine that the communications are no longer good, muting of the audio output will occur and the processors will attempt to reestablish communications on a different channel. Due to noise, there will be errors in the received signal. However, the DSAT code must still be detected even if some errors are present in the received signal. Otherwise, even minor noise or errors could repeatedly disrupt communications.
If a valid DSAT code is detected the audio output of the mobile radiotelephone will be enabled, but if the received DSAT code is not valid then the audio output of the radiotelephone will be disabled. The received DSAT code must be checked very rapidly to determine its validity so that the muting can be quickly released, or else the beginning of a spoken word will be cut off, and so that muting can be quickly applied, so that the listener will not hear noise for an excessive duration. Therefore, the validity of a received DSAT code must be determined within 175 milliseconds, which is 35 bit times, even when the input data stream may have a 3% bit error rate and the precise position of the start bit in the received DSAT code is not known. Furthermore, once a received DSAT code has been determined to be valid, it should not be declared invalid even if the bit error rate increases up to 10%. However, an invalid DSAT code should not be declared valid.
Therefore, there is a need for a DSAT code detector which can rapidly detect a valid DSAT code.
Furthermore, there is a need for a DSAT code detector which can tolerate a predetermined number of errors in the received DSAT code without declaring that the received DSAT code is invalid.